The present invention relates to systems and methods for stimulating the cochlea. More specifically, the present invention relates to systems and methods of stimulating the cochlea for representing fine time structure in incoming sounds.
Fine time structure (“FTS”) is the fast varying information present in sounds. The FTS has been shown to be essential for recognition of musical melodies, as well as for finely discerning other sounds.
In conventional, pulsatile, continuous interleaved sampler (“CIS”) strategy, the incoming sound is broken into a small number of frequency bands, for example, between 8 to 22 bands. A more complete description of an exemplary cochlear implant system is found in U.S. Pat. No. 6,219,580, incorporated herein by reference. Conventionally, the slowly-varying envelope is extracted from each band and this envelope information is used to modulate a high-frequency pulsatile carrier signal that is presented to a multiplicity of stimulating electrodes. In this conventional method of processing sound, some of the FTS information is discarded, namely, the fast-varying components present in each frequency band or stimulation channel.
FTS can be conveyed to the auditory nerve fibers in two different ways: (a) temporally, with respect to the discharge patterns of the auditory nerve fibers over time; and (b) spectrally, with respect to the cochlear excitation pattern over the length of the cochlea. Because different segments of the cochlea, over its length, are associated with different sound frequencies, the spatial location on the cochlea represents the spectral frequencies of the incoming sounds. It is unclear which of the two modes of variation, temporal or spectral, is more important for conveying the FTS.
In a conventional CIS system, the use of discrete electrodes fixes the position of the electrical stimulation field emanating from each electrode. Thus, if eight electrodes are used, there are eight dominant, but fixed, stimulation points on the cochlea. Spatial points on the length of the cochlea, which are between electrodes and between the dominant stimulation points, are poorly stimulated because only those areas closest to the electrodes are well stimulated. As a result, the conventional system can not accurately convey the FTS spatially to the cochlea.
It would be desirable to capture the FTS in incoming sounds and to deliver this information with improved spatial accuracy over the length of the cochlea. In addition, it would be desirable to achieve these results employing a stimulator system that is not capable of simultaneously delivering separately controlled current pulses through at least two electrodes, i.e., the pulse amplitude delivered from each electrode cannot be varied independently.
What is needed, therefore, is a method and system of processing auditory sound waves into electrical signals that captures FTS information and accurately directs this information to appropriate nerves that spatially innervate the cochlea.